Hydrolyzed dasheen starch flocculants

ABSTRACT

Dasheen starch hydrolyzed in the presence of insoluble metal salts formed in situ is a surprisingly effective flocculant for destabilizing dilute as well as thick sludge suspensions and is particularly effective when used in the treatment of bituminous tar sands tailings and in phosphate slimes.

This is a division of application Ser. No. 562,354, filed Dec. 16, 1983,now U.S. Pat. No. 4,502,960.

BACKGROUND OF THE INVENTION

This invention relates to a novel hydrolyzed dasheen starch compositionand the process for making and using the same. More particularly, thisinvention relates to the discovery that dasheen starch which ishydrolyzed in the presence of insoluble metal salts formed in situ is avery effective flocculant in destabilizing dilute as well as thicksludge suspensions.

In general, the suspensions destabilized by the starch composition ofthis invention are aqueous colloidal suspensions containing either clayminerals or metal oxides-hydroxides which are formed in large volumeduring mining operations in the recovery of such materials as coal,bitumen from tar sands, and metals and such suspensions are also formedin making paper operations. In the case of metal mining operations,suspensions known as slimes are formed, typically phosphate slimes orlike materials produced in the mining of copper, nickel, and titanium.In coal and tar sands minings, for example, the mining effluenttypically contains dilute or thick clay mineral suspensions. In papermaking processes ceebulosic fines, clays and other solids are present inthe large volumes of liquids obtained.

In order to properly dispose of these voluminous mining discharges,regardless of their source, flocculants have conventionally beenemployed to destabilize these suspensions and thus permit the effectiveseparation of water from solids. The use of wheat, corn and potatostarch which has been hydrolyzed in the presence of metal salts formedin situ has been disclosed in U.S. Pat. No. 4,289,540 which issued onSept. 15, 1981 and is hereby incorporated by reference. It has now beenfound that surprisingly superior flocculation is obtained if starch fromthe root of the dasheen plant is used for preparing the hydrolyzedstarch composition to give the starch flocculant.

OBJECTS OF THE INVENTION

It is, therefore, an object of this invention to provide an effectiveflocculant in destabilizing dilute as well as thick sludge suspensions,particularly colloidal suspensions obtained from mining operations.

It is a further object of our invention to provide an effectiveflocculating agent for treating tar sands tailing streams which carrysuspended clay particles.

It is another object of our invention to provide such a flocculatingagent which is economical to prepare and employ in the treatment of bothtar sands tailing streams, and phosphate slimes obtained from phosphatemining operations.

In another aspect, it is yet another object of our invention to providesuch a flocculant which is safe and easy to handle and which itselfoffers no ecologically undesirable side effects.

BRIEF SUMMARY OF THE INVENTION

In accordance with the present invention, it has been found that theforegoing and other objects may be achieved by employing a hydrolyzeddasheen starch as a flocculant in destabilizing dilute as well as thicksludge suspensions. More particularly, it has been found that when thesestarches have been hydrolyzed in the presence of metal salts formed insitu there is formed a surprisingly effective composition fordestabilizing such suspensions. Still more particularly, and especiallyeffective flocculant composition may be prepared when calcium aluminophosphate dasheen starch is used.

As aforestated, the sludges being treated in accordance with thisinvention are aqueous colloidal suspensions containing either clayminerals or metal oxides-hydroxides which are formed during miningoperations or other operations generating such suspensions. For sake ofillustration, an simplicity only, the following description will bedirected to colloidal clay suspension obtained from bituminous tar sandsmining. However, it will be understood that this invention is directedgenerally to the destabilization of all such suspensions.

PREPARATION OF THE HYDROLYZED STARCH

As aforementioned, the flocculant is prepared from a dasheen starchwhich has been hydrolyzed in the presence of inorganic salts formed insitu.

The dasheen starch itself is readily obtained from the dasheen plantwhich is the common name for the plant Colocasia esculenta, includingthe variety antiquorum (taro). The plants are among the few ediblemembers of the aroid family (Ayaceae) and are native to SoutheasternAsia and Malaysia. The edible corms (underground stems) from which thestarch is obtained support a cluster of large leaves 4 to 6 feet longoften called elephant's ears. Separation of the dasheen starch from theroot is well known in the art and need not be repeated in detail here.However, for the sake of completeness, the dasheen starch used in theexamples which follow was obtained in the following manner:

To 15 pounds of the peeled, diced dasheen corms 1.5 gallons of water wasadded to which 15 gms of potassium metabisulphite had also been added toprevent discoloration. The mixture was chopped with water in a blenderat low speed just long enough to break up the cells (approximately 2mins.). Then, the mass was filtered through a No. 60 mesh (250micrometers) filter to remove the fibrous material from the starchsuspension. The fibrous material was washed twice with fresh quantitiesof water and the starch suspensions combined and allowed to settle.Finally, the water was decanted and the starch was dried in an oven at60° C. or sundried after which time it was ready for use.

The hydrolysis of the dasheen starch is achieved by simply heating thestarch in the form of an aqueous suspension at about 85° to 95° C. andpreferably at about 90° C. in the presence of soluble metal salts whichwill form an insoluble salt in situ. The starch should desirably bepresent in an amount of 1 gm to 10 gm per 100 ml of water, andpreferably 2 to 6 gms per 100 mls of water.

Among the salts which may be employed during the hydrolysis are salts ofmetals such as sodium, potassium, ammonium, magnesium, calcium andaluminum. The respective anions may be sulfates, acetates, chlorides,nitrates, chlorates, bromides, iodides, thiocyanates, phosphates and thelike. Particularly effective for purposes of this invention, as appliedto tar sands tailing ponds, is calcium alumino phosphate dasheen starch,although such other salts containing sodium, aluminum and phosphate ionsand the like may also be used.

As indicated, the salts are added as soluble salts in order to form theinsoluble salt in situ. Thus, for example, the preferred calcium aluminophosphate salt is advantageously formed in situ by the addition ofspecified amounts of calcium hydroxide, aluminum sulfate, and sodiumphosphate to the aqueous dasheen starch hydrolysis medium. In any event,the resulting salt should desirably be present in amounts of about 10 to30 gms per 100 gms of starch, and preferably 15 to 20 gms per 100 gms ofstarch.

In the case of treatment of known phosphate slimes from phosphate miningoperations with the flocculant of this invention, it will be understoodthat the presence or addition of a phosphate such as sodium phosphate,is not necessary and may be omitted from the salt preparation.

If desired, the dasheen starch hydrolysate may be treated to removewater; e.g. drying by freeze drying, air drying, spin drying or thelike, to provide a powder which is convenient to handle, store andtransport, yet which can be readily redispersed in water at thetreatment site.

The following examples are provided for purposes of illustrating but notlimiting the scope of this invention with respect to the preparation anduse of the starch flocculants.

SYNTHESIS OF HYDROLYZED DASHEEN STARCH FLOCCULANT

Five grams of dasheen starch is weighed into a flask fitted forrefluxing. Two hundred milliliters of aqueous solution containing 0.617gm of Al₂ (SO₄)₃.18H₂ O, 0.704 gm Na₃ PO₄.12H₂ O, and 0.463 gm Ca(OH)₂is added to form calcium alumino phosphate in situ in the presence ofthe starch.

The suspension is refluxed as 90° C.±5° for two hours with simultaneousstirring. The hydrolysis is considered complete when the insolublestarch is converted into a colloidal solution. The volume is then madeup to 250 ml with distilled water, thus giving a stock solution of20,000 ppm of calcium aluminum phosphate dasheen starch.

A preferred procedure for making the hydrolyzed dasheen starch employsthe method described in U.S. patent application Ser. No. 327,850, filedDec. 7, 1981, which application is also incorporated herein byreference. In the preferred method, an aqueous suspension of the starch,alum and lime is heated until maximum viscosity of the suspension isobtained and immediately thereafter, the salt to be added (e.g.trisodium phosphate) is added to the mixture. The general procedure forsuch preferred method is as follows:

To 2.67 liters of tap water add 109.89 g dasheen starch, 10.18 g limeand 6.78 g alum and heat the mixture with stirring at a constant speedof 100 rpm at a rate of 0.5° C. per minute. Stirring is carried out witha Cole-Palmer Master Servodyne torque stirrer calibrated to giveviscosity value. As heating and stirring are continued the viscosity isobserved to rise significantly when a temperature of about 60° C. isreached and continues to rise until about 70° C. is reached, at whichpoint it begins to fall and then 15.46 g of trisodium phosphate (Na₃PO₄) is immediately added. Heating and stirring at the same rate iscontinued until a temperature of 85° is reached, and then the heat isremoved and the material is allowed to cool to room temperature. Thishydrolyzed dasheen starch suspension containing about 4% of the starchcomposition was used in the comparative tests which follow.

COMPARATIVE EXAMPLES

In order to compare and evaluate the efficiency of the dasheen starchflocculants as settling aids, the test flocculants were used to treattar sand tailings.

The tar sand tailings used contained 1.93% solids (w/w) which arelargely silts and clays. The fines are quite dispersed and tend toremain in suspension for a long period of time.

Settling Tests of Tar Sand Tailings Treated With Starch Flocculants

100 ml of the tar and sand tailings was poured into a 100 ml cylinderand then 0.1 ml of alum (0.06 m-mole/l) was added to the tailingssample. The cylinder was inverted 5 times to mix the tailings with thealum. Then 0.25 ml of the 4% hydrolyzed dasheen starch flocculantsuspension was poured into the cylinder. The cylinder was again invertedfive times. The dosage of treatment is approximately 100 PPM. The sameprocedure was used for a hydrolyzed wheat starch flocculant used forcomparison. The treated samples were then allowed to settle in thecylinders without further disturbance. The sediment heights wererecorded periodically until little change was observed.

4.3 Kynch Analysis of the Settling Tests

Results of the settling tests were evaluated in terms of Kynch analysiswhich gives a settling time called "ultimate time" (T_(u)). T_(u) is anindicator of the duration required for a sediment to reach close to theultimate height and this parameter is used to evaluate the efficiency ofsettling aids in suspensions.

Another parameter for the expression of settling efficiency of sedimentsis called "Unit Area" which is defined as follows:

    Unit Area=T.sub.u /(C.sub.o ×H.sub.o)

where

C_(o) =initial solids concentration (ton/cu. ft)

H_(o) =initial height (ft)

T_(u) =ultimate time (days)

Unit Area is expressed as sq. ft/ton/day. This is the area required tosettle one ton of solids per day. Small Unit Area indicates efficientsettling. The T_(u) and unit areas of the hydrolyzed dasheen starchflocculant treated samples, a hydrolyzed wheat starch flocculant treatedsample and an untreated sample are presented in Table I.

                  TABLE I                                                         ______________________________________                                        Tar Sand Tailings Treated With Flocculants                                                         Unit Area                                                              T.sub.u (Min)                                                                        (sq. ft/ton/day)                                         ______________________________________                                        Untreated       1440     284.63                                               Hydrolyzed Wheat                                                                              98       191.73                                               Starch Flocculant*                                                            Hydrolyzed dasheen                                                                            40        78.27                                               Starch Flocculant*                                                            ______________________________________                                         *Prepared by the preferred method of S.N. 327,850                        

As can be seen from the above table the hydrolyzed dasheen starchflocculant of the invention is surprisingly superior to the hydrolyzedwheat starch flocculant.

The invention claimed is:
 1. A composition comprising a hydrolyzeddasheen starch obtained by the aqueous hydrolysis of dasheen starch inthe presence of about 10 to about 30 gms per 100 gms of starch ofinsoluble metal salts formed in situ, and wherein the salts employedduring the hydrolysis to form said insoluble salts are the soluble saltsof metals selected from the group consisting of sodium, potassium,ammonium, magnesium, calcium and aluminum, and the respective anions ofsaid soluble salts are selected from the group consisting of sulfates,acetates, chlorides, nitrates, chlorates, bromides, iodides,thiocyanates, and phosphates.
 2. The composition according to claim 1wherein the salt contains calcium, aluminum and phosphate ions.
 3. Thecomposition of claim 1 wherein the aqueous hydrolyzed starch is dried toform a solid composition.